What numerical CRF values do the assorted output qualities correspond to in the ffmpeg video encoding options
A suggestion from a person on IRC led me to examine the encoded streams which conveniently contain a SEI NAL with the encoding options.  Based on encoding streams with varying output qualities it seems

CRF
32   lowest
29   very low
26   low
23   medium
20   high
17   perceptually lossless


and the lossless option is a different mode with qp=0 which activates the lossless mode of H.264 (which probably uses a PCM mode according to what little I have read).  I did not bother to study the CBR mode.